Understanding the Doppler RI: Impact of renal arterial distensibility on the RI in a hydronephrotic ex vivo rabbit kidney model

The aim of this study was to evaluate the influence of elevated ureteral pr essure on renal arterial distention, and thereby on the Doppler resistive i ndex. Seven isolated rabbit kidneys were subjected to a pulsatile perfusion while the renal pelvis was pressurized via the ureter. Renal vascular pres sure, flow, resistance (pressure/flow) and conductance (flow/pressure) were compared to simultaneous resistive index measurements using linear regress ion analysis. Changes in the Conductance Index (systolic conductance - dias tolic conductance/systolic conductance) were likewise compared to the resis tive index. Elevations in ureteral pressure were significantly correlated w ith (1) increased resistive index values, (2) increased mean renal vascular resistance, (3) decreased mean conductance, and (4) increased conductance index values (for all correlations P < 0.05). The increases in the resistiv e index correlated significantly with increases in the conductance index. T his study shows that elevated ureteral pressure, likely acting via intersti tial pressure, diminishes the conductance of the renal vascular bed. Becaus e this effect is more dramatic at diastole, the cyclic patterns of flow are altered, resulting in elevated resistive index values. The results indicat e the importance of the interaction between vascular distensibility and pul satile flow, rather than overall mean renal vascular resistance, in determi ning resistive index values.